Field of the Invention
The present invention generally concerns apparatus and systems for drilling wells, such as for production of petroleum products and more specifically concerns methods and systems for ensuring efficient well drilling and protection of well drilling systems during drilling operations. More particularly, the present invention concerns a closed loop control system for drilling rig controls, which is responsive to downhole measurement by drilling tools. The measured downhole data is transmitted by measurement while drilling (MWD) telemetry to a digitally controlled switching control regulator (SCR) module via an interfacing computer and utilized to refine the drilling controls by automated correction of driller inputs to the drilling controls of the well drilling system.
For production of petroleum products, such as crude oil, natural gas and mixtures thereof from subsurface reservoirs boreholes are drilled in the earth from the surface to one or more subsurface petroleum bearing zones, typically by rotating a drill bit against the formation. The drill bit may be rotated against the formation by a rotary table or top drive of a drilling rig via multiple interconnected lengths or stands of drill stem to which the drill bit is connected. Alternatively, the drill bit may be driven by a downhole motor, typically referred to as a xe2x80x9cmud motorxe2x80x9d which is connected to the drill stem or to coiled tubing and which has a rotary drive shaft to which the drill bit is connected. Regardless of the character of the drilling system, the drill stem or coiled tubing defines a flow passage through which drilling fluid, typically referred to as xe2x80x9cdrilling mud,xe2x80x9d is pumped. The drilling fluid is typically a weighted slurry which, even in absence of pump pressure, develops sufficient bottom hole pressure to overcome formation pressure and prevent well blowout in the event a pressurized subsurface pocket is encountered by the drill bit.
A well drilling device, which is typically referred to as a xe2x80x9cdrilling rig,xe2x80x9d for drilling with interconnected lengths of drill stem, is provided with a controllable drill stem handling apparatus including a crown block and a traveling block each having multiple sheaves about which wire cable is laced. The traveling block is typically provided with a hook which typically has supporting engagement with the bail of a swivel apparatus which permits rotation of the drill stem or a rotary table driven kelly to which the drill stem is connected and provides a fluid inlet through which drilling fluid is pumped into the drill stem by one or more mud pumps. The wire cable is fed from a storage spool of a drilling rig drawworks to the sheaves of the crown block and traveling block and provides for supporting, controllably lowering or raising the traveling block and thus the drill stem to thus control engagement of the drill bit against the formation as the drill bit is rotated during drilling. Alternatively, where rotation of the drill stem is accomplished by a top drive system, the top drive mechanism and the swivel assembly are supported, lowered and raised by the hook of the traveling block.
Personnel accomplishing actuating control of the drilling rig is typically an experienced person known as the xe2x80x9cdrillerxe2x80x9d. During most phases of rig operation the driller is stationed at a control console which is equipped with a display or multiple displays identifying the various important parameters of the well drilling operation. The wire cable storage spool of the drawworks typically incorporates a brake which is controlled by the driller or by a software program commanded by the driller, permitting controlled payout of wire cable from the spool and thus permitting controlled weight actuated descent of the traveling block and drill stem for controlled penetration of the drill bit into the formation.
As the true objective of rig controls is to achieve a particular set of drilling parameters downhole and at the bit, if the actual measurements of the downhole drilling parameters are not available, one has to compute their values from the surface measurements only. A typical case is to compute the Downhole Weight On Bit (DWOB) from the total weight suspended to the Derrick (Hook load), by subtracting the weight of the pipes, which are suspended in Tension (Wt). This calculated weight on bit is commonly called Surface Weight On Bit (SWOB). Hookload and SWOB are basically related by the following equation:
SWOB=Hookloadxe2x88x92(Wt)xe2x80x83xe2x80x83(1)
The difference is equal to the sum of all the pipes or drill collars, which are below the neutral point of tension/compression (usually the drill collars).
Immediately, some complications become apparent, which can be alleviated by downhole measurements:
Effect of Inclination:The pipes, which are not in tension only, contribute to DWOB through the component of their weight, which is aligned with the borehole, not by their absolute weight. Hence a first complication of the equation:
SWOB=Hookload(Wt)xc3x97Cosines(Inclination)xe2x80x83xe2x80x83(2)
Effect of Flotation in Drilling Mud:
The drill string is immersed in the drilling mud, which has a significant density (pMud), resulting in a flotation force proportional to the weight of fluid displaced by the immerged part of the drill string. Hence a second complication of the equation, with Vstring of the immerged part of the drill string
SWOB=(Hookload(Wt)xc3x97Cosines(Inclination))pMudxc3x97Vstringxe2x80x83xe2x80x83(3)
This being a first approximation, given to illustrate the actual complexity of the problem, as the floatation force is vertical, and the drill string may be inclined on a significant part of its length, requiring the knowledge of the well profile (Inclination versus depth) for exact calculation.
The Third Effect is Friction of the Drill String against the Borehole (F):
The friction force is opposed to the direction of the displacement. As the driller can move the drill string up and down when the bit is off-bottom, it is possible to have a surface measurement of the friction forces:
Fric=xc2xd(Hookload going up Hookload going down)xe2x80x83xe2x80x83(4)
This reduces the actual weight on bit, and can be accounted for in the calculation of SWOB:
SWOB=(Hookload(Wt)xc3x97Cosines(Inclination))pMudxc3x97Vstring Fricxe2x80x83xe2x80x83(5)
As drilling of a well progresses, the friction forces can change for several reasons:
inclination changes, coefficient of friction changing as new formations are cut or as the borehole degrades, packing of debris around the drill string, friction of stabilizers increasing when the hole size decreases as the drill bit wears down or when the borehole collapses. The only way to actualize Fric, if no downhole measurements are available, is to stop drilling and repeat the up and down motion to obtain a new value of the difference. Since this activity results in interruption of the drilling process, it is not done frequently. Whereas, the Hook load is constantly adjusted manually by the driller when drilling a 90 ft stand, generally, the up and down motions only occur when connecting a new 90 ft stand. The estimation of Friction is therefore established at each connection, however, thereafter assumed constant when drilling the next 90 ft section.
One can readily identify a number of scenarios where a driller""s manual control input based on experience will fail to accomplish the desired result:
Scenario 1: Stabilizer Hanging Up
If one stabilizer of the drill string is hanging up, the weight of the drill string is not transmitted to the drill bit, and lowering the block (traveling block hook supporting the drill string) to achieve a constant rate of penetration (ROP) will not have the desired effect. In reality, it can cause damage to the drill string by buckling and other consequences of overload.
Scenario 2: Sudden Reduction in Formation Strength Due to Pressure Imbalance Between Mud and Formation, or Properties of Rock Geomechanics
If the driller maintains the same SWOB command setting, the ROP will suddenly increase at a time where it may be critical to slow down and analyze the situation.
When using an automatic drilling control process strictly based on surface information, similar limitations affect the computer model. In the absence of downhole data, it assumes that the relation between SWOB and DWOB is constant for a certain length of time. Consequently, the automation has been limited to simpler applications such as maximum and minimum block height, or maximum block speed.
It is a principal feature of the present invention to accomplish automated control of the downhole weight on bit while drilling, as well as controlling other well drilling functions such as downhole and surface torque control, downhole pressure control and MWD automatic frequency selection in response to downhole data.
It is another feature of the present invention to provide for downhole measurement responsive closed-loop control of various well drilling functions, by acquiring selected downhole parameter measurements by means of an MWD tool component of a drill string, transmitting the digital data output of the MWD tool to the surface via MWD telemetry and inputting the digital data to the rig controls computer as it becomes available via telemetry for updating the mathematical model of the drilling control system response.
It is another feature of the present invention to update the mathematical model of the drilling control system at frequent intervals during drilling, with data representative of measured downhole drilling parameters that are sensed during drilling.
It is an even further feature of the present invention to accomplish well drilling using a drill string having a top drive or downhole drilling motor being controlled by a drawworks that is controlled by a mathematical model programmed into a drilling control system and with the mathematical model being updated or calibrated frequently with substantially real time downhole data representing drilling parameters at or near the drill bit, so that automated optimized drilling is accomplished.
Whereas downhole measurements transmitted by MWD tools are known in the drilling industry, in the past data representing downhole measurements have only been used to provide a human operator (the driller) with additional information indicating downhole conditions, thereby permitting the driller to manually adjust the rig controls more in response to actual downhole conditions rather than relying on interpretation of downhole conditions from surface measurements.
The recent deployment of digitally controlled SCR modules offers the possibility to refine the rig controls by supplementing the driller with automatic corrections to minimize classical control problems such as overshooting the desired control level, oscillations around the desired control level, late response, out of phase response, or erroneous control input. Currently, digital SCR modules use surface measurements in computer models to achieve automatic corrections to manual or driller commands by correcting or optimizing the driller inputs to the rig controls. However, because that is done without direct knowledge of downhole conditions, the automation has been limited, thus, still depending on operator skills.
According to the principles of the present invention, by digitally connecting the relevant downhole measurements to the computer of a digital SCR module or other automated drilling system, the drilling system software model can be calibrated with actual downhole data. Measurement data reflecting surface conditions and measurement data reflecting downhole conditions during drilling are compared by the mathematical drilling control model of the digital SCR module and used to update the system software model with comparative surface/downhole data or with the downhole data. The calibrated software thus recognizes manual control commands that are optimized by measured downhole conditions. When this condition occurs, the drilling control software causes the manually input commands to be overridden or optimized, thus permitting drilling to continue in response to actually measured downhole conditions.
To achieve the next step in automation in controlling the weight on bit while drilling and the rate of penetration, the proposed invention uses all relevant downhole information to update the rig controls computer model as frequently as they become available through the MWD telemetry. In the past the rig controls computer model has been updated at the time another 90-ft section of drill pipe is connected to the drill string, for example at each 90-minute interval, when drilling is progressing at the rate of 90 ft per hour. The present invention permits the software to be updated or calibrated once each minute or so d drilling, and without necessitating interruption of the drilling operation to accomplish calibration.
The downhole measurements are processed by the computer interface/transfer function of the MWD surface acquisition system and are output in digital form. The digital downhole measurement data is then sent to the digital SCR module as shown in FIG. 1. The transfer function that is used in the control module software is updated by comparing data representing downhole measurements and data representing surface measurements. For example, the traveling block height is no longer servoed from SWOB, block speed, and stand pipe pressure (all surface measurements) but can also use, as non-limitative example, DWOB, downhole internal pipe pressure, and annulus mud pressure (all available from current MWD tools).
The update rate required from the MWD telemetry is not necessarily faster than current capability, as the updates are primarily used to update the mathematical model-of the system response (the transfer function), not to directly change the rig control settings. Therefore, even one update per minute is a significant improvement over the current one update per 90 ft stand connection (typically one hour when drilling at 90 ft/hr). The MWD telemetry link is bidirectional, thus permitting operational commands to be transmitted downhole to the MWD tool and to any associated downhole equipment.